In these networks it is necessary to provide additional optical power for the oscillator associated with each receiving terminal which provides the power for the mixing action in the photodiode of the receiving terminal, and which acts as the primary means of channel selection by frequency tuning in a multi-channel network. Optical power is also required for the modulator. The efficient provision of optical power is therefore a key requirement, particularly for wideband systems to which many terminals are connected. The requirements are particularly demanding in a coherent system where local oscillator power is needed and many more terminals are possible.
The most widely used method of providing optical power is to associate an individual laser with each terminal. Although technically simple, the expense of providing a laser for each terminal and the potential maintenance costs make this solution unattractive. An alternative is to site a single more powerful laser, or a bank of lasers, at a central point in the network and to distribute the optical power along the same optical fibres as the information carrying signals. Such an arrangement shares the laser power and simplifies maintenance but necessitates routing the optical power through many isolators, polarization controllers, combiners and other optical components. Together with the fibre, these introduce considerable optical loss and can absorb a significant part of the optical power.
In accordance with the present invention, an optical fibre network has at least one transmitting station and a plurality of receiving terminals connected to the transmitting station by monomode optical fibres; and one or more sources of temporally coherent optical power connected to the network to feed optical power locally to the receiving terminals, wherein the number of sources is less than the number of terminals.
This invention makes use of the important properties of monomode optical fibres which is that the losses associated with such fibres are much lower than with multimode fibres. This enables optical power to be fed locally to a number of terminals so that the optical losses involved are only those associated with the local optical fibres and not the main network. Furthermore, the number of sources is reduced from one per terminal.
In some cases, each source may comprise a plurality of subsidiary sources for supplying optical power of respectively different wavelengths; and selection means for selecting one of the subsidiary sources to be connected to the network.
This is particularly useful in wideband networks where signals with different wavelengths are passed along the network and enables the correct channel to be received by a receiving terminal.
In general, each source or subsidiary source will comprise a laser transmitter. The selection means may comprise an optical space switch.
In many networks the receiving terminals are arranged in one or more limbs of series connected groups of terminals and in this case each source of optical power may be connected to a respective one or more of the groups.
The networks may take the form of a star or tree distribution structure or a combination of both.
When the receiving terminals are connected in this way, there are a variety of methods by which the sources may be connected to the network. For example, at least one source may be associated with a respective terminal, whereby the source delivers optical power to the associated terminal and to other receiving terminals in the same group downstream of the associated terminal. Conveniently, the associated terminal is positioned at an upstream end of the limb so that all receiving terminals in the same limb are supplied with optical power from the same single source.
Preferably, a multi-port beam splitter (for example with four, six or eight ports) is used to connect terminals to the network. A multi-port beam splitter is hereinafter defined as having a plurality of optical fibres adjacent one another, adjacent ends of the fibres defining two sets of ports and which is arranged such that optical signals received by one port of a set are transmitted in equal or unequal proportions from the other set of ports.
Preferably, all the receiving terminals of a respective limb are connected to the limb by respective four port beam splitters, as hereinbefore defined, first ports of the first and second pairs (sets) of ports facing upstream and downstream of the limb respectively, and a second port of the second pair (set) of ports being connected to the terminals, wherein the receiving terminal associated with a source has a second port of the first pair of ports connected to the source whereby optical power is delivered to the associated receiving terminal via the second port of the second pair and to a downstream receiving terminal via the first port of the second pair.
With this arrangement, each four port beam splitter downstream of the associated terminal receives signals from the transmitting terminal and optical power from the local source at its first port of the first pair and this is passed to the respective terminal via the second port of the second pair while the remaining portion of the signals and optical power are passed to further downstream terminals via the first port of the second pair. The proportions by which the power and signals are split between the output ports depends upon the form of the beam splitter but typically will be 50%.
If six or eight port beam splitters are used, more than one terminal could be connected to the network via different ports of the same splitter.
In another example, at least one source is connected to one or more limbs via additional coupling means spaced from the receiving terminals. For example, one source could be connected to two limbs of series connected groups of receiving terminals upstream of all the terminals in each group. Alternatively, an upstream end of one limb may connected to a mid-portion of another limb.
Clearly, there is a variety of ways in which sources may be connected to limbs and in general this will be decided by the number and physical arrangement of receiving terminals and the number and power of sources available.
Conveniently, the coupling means comprise one or more multi-port beam splitters, as hereinbefore defined.
In a further example, in which each receiving terminal of a limb is connected to the limb by a multi-port beam splitter, as hereinbefore defined, a second port of the first set is also connected to the terminal, and the associated source is arranged to feed optical power along the limb from a downstream end of the limb, whereby optical power is supplied to each terminal in the limb via the second port of the first set.
In this example, optical power travels along the limb in the opposite direction to signals from the transmitting terminal.
It should be understood that in some cases receiving terminals may also have the capability to transmit signals.